The invention relates to a system for the display of light images.
It can be applied more particularly to the projection of video images on a wide screen.
Wide-screen video projection systems, based on liquid crystal active matrices are currently being developed. These matrices generally use "nematic helix" type liquid crystals associated with thin layer transistors for the addressing of the different pixels by the video signal. They are the equivalent of dynamic slides. The resultant projectors can be likened to slide projectors by virtue of their design. The color images are obtained either from three monochromatic optic valves or from a single valve provided with filters colored according to an arrangement similar to that of color TV tubes.
These projectors are very attractive in principle but have the drawback of having a very poor light efficiency of a few per cent. For, the cells used make it necessary to work between crossed polarizers which means, firstly, that one of the components of polarization of the illumination source is not used (entailing a 50% loss) and, secondly, that there is an additional absorption due to the polarizers used that has to be taken into account. The useful surface of each pixel is small (because of the distance between each pixel, the area occupied by each transistor etc.) and will be all the smaller as it is sought to set up, for identical cell areas, very high-definition cells compatible with HDTV.
Although contrivances enabling an increase in the light efficiency of such projectors are currently being studied, it is unlikely that it will be possible to make projectors with an efficiency of more than 10%.
In parallel with the use of liquid crystal matrices, trichromatic laser projection based approaches can also be considered. Approaches such as these use methods for the XY deflection of laser beams by optomechanical (including holographic) and acousto-optical methods. These approaches are limited by several factors:
flickering phenomena (due to the non-remanence of the screen); PA1 the difficulty of making fast deflectors with a large number of dots while at the same time preserving high efficiency; PA1 the existence of parasitic phenomena due to speckle noise (related to the coherence of the laser sources used). PA1 a spatial modulator of light receiving a first light beam and retransmitting a modulated beam; PA1 an energy transfer device receiving the modulated beam and a second light beam and transferring the energy from the second light beam towards the modulated beam.
The approach proposed is based on the use of liquid crystal matrices (the industrial development of which is inevitable) in a projection system that enables efficient energy transfer between a signal beam and a pump beam.